1. Field of the Invention
The present invention relates to a color conversion method of converting image data for printing to image data for a proofer, a color conversion apparatus and a color conversion definition storage medium for storing therein a color conversion definition to perform such a color conversion, which are applied to a system wherein when a color printing is performed by a color printing machine using a spot color ink as well as a process color ink, a proofer such as a printer or a CRT display unit is used to output a proof image representing a color of a color printed matter (including print-out and image display).
2. Description of the Related Art
Hitherto, in the event that a color printing machine is used to perform a color image printing, before the printing is performed, a proof image, which is similar in color to the utmost to the same color as a color of an image printed by the color printing machine, is printed out using a color printer and the like, or alternatively displayed on a screen of a color display unit. In the event that such a proof image is produced, there are detected a print profile describing a relation between image data and colors of actual printing matters, which print profile is associated with a type of a printing machine for the intended printing and using conditions of the printing machine (conditions necessary for some printing, including a sort of ink to be used and a quality of a printing paper; and a type of a printing machine, are referred to as printing conditions), and a proofer profile describing a relation between image data and colors of actually outputted proof images, which proofer profile is associated with a type of a proofer outputting proof images and using conditions of the proofer (conditions necessary for outputting of some proof image, including a type of a proofer, are referred to as proof conditions), and then the image data for printing is converted to the image data for the proofer in accordance with the print profile and the proofer profile, so that a proof image is outputted in accordance with the converted image data for the proofer. Thus, it is possible to obtain the proof image that coincides with the actual printing matter in color. Usually, the print profile associated with the typical printing conditions is offered from a printer (a printing service trader), and the proofer profile is also offered, with respect to printers (devices) for the purpose of a proof image output, from a maker of the printers.
In the event that the image data for printing is converted into the image data for a proofer to output the proof image, usually, the print profile and the proofer profile are combined to produce a LUT (Look Up Table), and the LUT is referred to and regarding a low order bit side on which the LUT fails to describe the association, an interpolation arithmetic operation is performed, so that the image data for printing is converted into the image data for the proofer.
Here, printing is performed usually using process color ink of four colors of CMYK, but in some case it happens that spot color ink is used in addition to the process color ink of those four colors. In this case, it is not realistic that there is prepared a print profile on the combination of the process color ink of four colors of CMYK and the various sorts of spot color ink in its entirety, and usually, there is prepared a print profile related to the printing using the process color ink of four colors of CMYK. Accordingly, the LUT thus produced is an LUT in which process color image data defined by color spaces of four colors of CMYK wherein the process color ink of four colors of CMYK is used to perform printing is converted into image data defined by color spaces of three colors of RGB wherein color materials of, for example, three colors of RGB is used to output a proof image for proofer.
In the situation as mentioned above, when the printing is performed using the spot color ink as well as the process color ink of four colors of CMYK, it is a problem as to how the proof image of the printed matter is outputted.
Usually, a printer for a proof image output is of a type of outputting an image through for example, color materials of three colors of RGB or color materials of four colors of CMYK, and it is extremely rare that spot color materials coincident with spot colors intended for use of the printing are prepared. Further, also in the event that a proof image is displayed on a CRT display unit, there is no way other than all the colors being represented by a combination of three colors of RGB. Thus, the printed matter including the spot color will be represented by three colors of RGB in its entirety.
In this case, according to the earlier technology, there is adopted a method in which spot color image data is converted into process color image data of four colors of CMYK in accordance with characteristics (coordinate values on the L*a*b* space and the like) of characteristic ink to be used for intended printing, which is available from a maker of the characteristic ink, and image data (dot % data and the like) for spot color for printing, the process color image data for spot color and the process color image data for process color ink of four colors of CMYK except the spot color are combined, and the combined process color image data of four colors of CMYK is converted by the conversion system such as the LUT into, for example, image data of three colors of RGB, for a proofer, so that a proof image is outputted based on the image data for the proofer thus converted. In this case, the spot color is separated into four colors of CMYK, and is converted into the image data for the proofer by the conversion system comprising the LUT as mentioned above for the process color. Thus, this is a problem in accuracy of color reproduction of the spot color.
In Japanese Patent Application Laid Open Gazette Hei. 10-248017, there is proposed a technology in which accuracy of color reproduction of the spot color is improved. According to the technology, of the printing image data, the process color image data of four colors of CMYK is converted into process color image data for a proofer by a first conversion system comprising the LUT as mentioned above, and on the other hand, as to a spot color, there is prepared a second conversion system adapted for the spot color, and image data for the spot color is converted into image data for the proofer by the second conversion system. Both the image data converted by the first and second conversion systems are combined on a color space (e.g. RGB color space) for the proofer.
According to such a technology, as compared with the above-mentioned earlier developed technology in which the spot color is separated and is converted by the conversion system which is not prepared as one for a spot color, there is prepared a second conversion system adapted for the spot color as well as a first conversion system adapted for the process color ink of four colors of CMYK. Thus, it is possible to perform a color conversion with greater accuracy as to the spot color and thereby improving the color reproduction accuracy of the spot color.
Now let us consider a case where there is constructed such a system that various types of proofers for a proof image output exist, and of the various types of proofers, a proofer for outputting a proof image is selected in accordance with, for example, speed and cost for producing the proof image. That is, for example, a system in which a proof image is displayed on a display screen of a CRT display unit and is printed out by a printer as a need arises. In addition, for printing out, there are prepared a plurality of printers, one of which is a printer for proof image output that is selected in accordance with speed and cost for print output, or necessary image quality. In this case, according to the technology proposed in the above-mentioned Japanese Patent Application Laid Open Gazette Hei. 10-248017, there is a need to do over again producing both the first conversion system and the second conversion system for each type of the proofer. This is a problem in efficiency of data management. Alternatively, it is possible to prepare both the first conversion system and the second conversion system for each type of the proofer beforehand. However, each of the first conversion system and the second conversion system includes an extremely large LUT which needs a large capacity of memory. A preparation of the conversion system requiring such a large capacity of memory in accordance with a type of the proofer needs a very large capacity of memory. This is a problem from the view point of structure of the system and the cost.